The present invention relates generally to seals.
Advanced general aviation propulsion engines capable of operating efficiently and reliably on new aviation fuels while producing emission and noise levels lower than those of current propulsion systems are needed to expand the role of light, business, and personal transportation aircraft and to revitalize the U.S. general aviation industry. Lighter, smaller, more efficient, consistently high performance, and low maintenance engines which will deteriorate little over time due to component wear are required to meet the need. The higher pressure ratios, mass flow rates, and operating speeds and temperatures needed to meet these conditions will place severe constraints on bearing and seal systems
My U.S. Pat. Nos. 5,833,369 and 5,902,049, which are incorporated herein by reference, disclose high load capacity thrust and journal bearings respectively for meeting the above need. The journal bearing comprises a smooth foil which is positioned to face the rotating shaft for relative motion therebetween by means of an air or gas or process fluid hydrodynamic film developed therebetween during such rotation. A corrugated sheet is disposed between the smooth foil and the bearing housing or sleeve for compliantly or resiliently supporting the smooth foil, i.e., bumps or ridges on the sheet deflect under load during shaft rotation or excursions. The elastic deflection of the sheet provided the clearance geometry required to generate the load-carrying hydrodynamic film. The thrust bearing comprises a smooth foil which faces a thrust runner and a corrugated sheet underlying the smooth foil for compliantly or resiliently supporting the smooth foil. Such hydrodynamic bearings are used in a wide variety of rotating machinery including, for example, air cycle machines, cryogenic turbo-expanders, and gas turbine engines.
Labyrinth seals, which use a series of teeth or blades in close proximity to a rotating shaft to break down pressure, allow high rates of gas leakage and are intolerant of rotor excursions and are thus insufficiently effective to meet the above need.
Noncontacting compliant hydrodynamic seals have been suggested to address the need for more robust long-life seals that can handle the expected temperatures, speeds, and pressure drops with low leakage during all facets of engine operation. See R. Flower, xe2x80x9cBrush Seal Development Systems, AIAA Paper 90-2143, 1990. In early experiments, compliant foil seals have demonstrated lower leakage rates than labyrinth seals and even brush seals, without the wear limitations. However, brush seals undesirably rub during large rotor excursions at full operating speed and thus fail to achieve the needed performance and life benefits throughout the expected operating envelope.
U.S. Pat. Nos. 5,100,158; 5,370,402; and 5,632,493 disclose pressure balanced compliant seals which utilize fingers which are cantileverly attached to swing toward and away from the shaft. It is claimed in the ""493 patent (column 7, lines 7 to 11) that the hydrodynamic forces caused by shaft rotation further increases film stiffness between each of the fingers and the shaft and that these hydrodynamic forces further enhance the ability of the fingers to dynamically track shaft motions due to runout and fast radial excursions. It is claimed that these seals can handle relatively large shaft excursions, e.g., 0.035 inch (See the ""402 patent at column 8, lines 7 and 8). Such a cantilevered type spring seal is undesirably structurally complex, fragile, and has low hydrodynamic load capacity. The structural compliancy of this type of construction depends solely on the staggered cantilevered foils and their interaction with each other visa-vis friction. For a given design dimension (such a the fingers"" length, width, thickness, etc.) there is stress limitation related to the levels of deflection one can impose upon such a cantilevered type foil, undesirably limiting the excursion capabilities of these seals. Moreover, the construction of this leaf (or finger) type seal undesirably interrupts the continuity of the hydrodynamic film, degrading appreciably the seal""s hydrodynamic performance.
U.S. Pat. No. 4,114,248 relates to a method of making resiliently coated metallic finger seals. U.S. Pat. No. 5,769,604 relates to a rotary seal assembly wherein first and second members (one of which is a rotor and the other of which is a stator) have annular sealing faces with two sets of grooves (one radially inwardly of the other) in one of the sealing faces. Pressurized fluid flows between the sealing faces and within the grooves thereby forming a gas film seal between the sealing faces. This seal assembly is said to have high angular compliance. The seals of these patents are not able to handle as large of shaft excursions as desired.
It is accordingly an object of the present invention to provide a seal which is effective (provides a small clearance), has low wear, and is capable of operating reliably even in the presence of large rotor excursions.
In order to provide such a seal, in accordance with the present invention, a seal comprises at least one smooth resilient foil for facing a rotor and at least one bump foil (one having a series of undulations circumferentially thereabout) for providing stiffness and controlled deformation to the smooth foil.
The above and other objects, features, and advantages of the present invention will be apparent in the following detailed description of the preferred embodiment of the invention when read in conjunction with the appended drawings wherein the same reference numerals denote the same or similar parts throughout the several views.